Water-soluble macromolecular substances with spectral properties of humic-like substances (HULIS) were recently found to form the major identified fraction of the organic aerosol at urban and rural sites in Europe. With primary sources identified so far (e.g., biomass combustion) it is not possible to explain the observed HULIS levels in Europe, therefore there is an ongoing search for other sources - which form HULIS in situ in the atmosphere. Here we show that secondary aerosol formation of atmospheric polymers occurs by heterogeneous reaction of isoprenoid or terpenoid emissions in the presence of a sulfuric acid aerosol catalyst. Competing oxidants such as ozone or the presence of humidity decreased the reaction yield, but the formation of humic--like substances was not disabled. Calculations indicate that the presented reaction pathway could be an additional source for HULIS in the continental aerosol.